Recently, an ultrasonic sensor is mounted to a vehicle to measure a position, a direction or a distance of an object around the vehicle.
The ultrasonic sensor is mounted to a sound receiving member like a bumper, a body or a head light. When an ultrasound is transmitted to the object from a transmitter which is formed separately from or together with the ultrasonic sensor, the ultrasound is reflected by the object and received by the sound receiving member. The sound receiving member accordingly vibrates and the ultrasonic sensor detects the vibration.
JP 2002-58097A proposes a piezoelectric substance as the ultrasonic sensor. In JP 2003-284182A, a membrane type ultrasonic sensor is proposed as the ultrasonic sensor.
The membrane type ultrasonic sensor includes a semiconductor substrate, at a surface of which a recess is formed. A portion of the semiconductor substrate corresponding to the bottom of the recess is thin and is therefore serves as a membrane. Thus, the membrane type ultrasonic sensor has a shape similar to that of a semiconductor substrate (for example, a semiconductor pressure sensor) having a diaphragm on its surface.
The membrane type ultrasonic sensor detects the ultrasound based on a vibration of the membrane. The membrane is proposed to be a thin film made of a piezoelectric substance or a capacitance type membrane including a detection electrode similar to a fixed electrode or a movable electrode.
The membrane type pressure sensor is attached, similarly to the semiconductor pressure sensor, to the sound receiving member at a surface on which the recess is formed. This is because the surface is almost flat except for the recess.
In this case, the recess is positioned between the sound receiving member and the membrane. The vibration of the sound receiving member is therefore transferred to the membrane through the air in the recess. A transferring path of the vibration from the receiving member to the membrane thus becomes longer by an amount of deepness of the recess. Efficiency in transferring the vibration may accordingly get worse.
In the case that the detection electrode is formed at a front side of the membrane not facing the recess, the efficiency in transferring the vibration becomes higher when the vibration is transferred faster to the front side than to a rear side of the membrane opposite to the front side.
In view of this, the sound receiving member may be simply attached to a front surface of the semiconductor substrate at which the front side of the membrane is formed.
However, as described in JP 2003-284182A, the surfaces of the membrane are uneven because of a wiring, a terminal or the electrode. It is therefore difficult to directly attach one of the surfaces of the membrane to the sound receiving member.
In addition, the membrane which is thin and fragile may suffer damage when the surface of the membrane is attached directly to the sound receiving member.